Carrier Sense Multiple Access (CSMA) is a well-known media access mechanism, which is used for example in Ethernet LANs (IEEE Standard 802.3) and wireless LANs (IEEE Standard 802.11). These network technologies are commonly used in networks that comprise shared media whereby multiple nodes simultaneously have access to the same media. The media may be any physical medium that can be simultaneously shared by many nodes, such as a cable, RF, powerline, etc.
Several examples of prior art shared networks are shown in FIGS. 1A to 1D. Referring to FIG. 1A, the network, generally referenced 10, comprises a plurality of communication nodes 12 (nodes A through E) that are connected to a common physical media. Each node 12 is connected to a shared media 14. An example of a shared media is the AC powerline wiring grid found in homes, offices and factories. In a residential environment, groups of neighboring residences are coupled together via the outdoor wiring, thus forming a huge common media. Note that the powerline media remains shared until reaching a transformer where signals cannot easily propagate beyond without signal couplers.
A shared media such as the powerline is typically characterized by a large variety of different signal propagation conditions. In many cases, portions of the media are invisible from other parts creating hidden node situations.
In many cases, a home, enterprise or other premise includes more than one communication network. Each communication network may be made up of a plurality of nodes with each network comprising at least two nodes. All nodes of the same network implement the same communication technique and are able to communicate with each other thus permitting interoperability (assuming that the propagation conditions over the media enable communication). Nodes from different networks may implement different communications techniques, in which case they are not able to communicate with each other. In addition, the propagation characteristics of the shared media (e.g., the powerline grid) may have large variations and irregularities. This results in large variations in the attenuation over the communication path between two given nodes. In addition, the propagation characteristics of the shared media (e.g., the powerline grid) may have large variations and irregularities. This results in large variations in the attenuation over the communication path between two given nodes.
Since the powerline grids of neighboring residences are physically connected via the power distribution network, the common media of FIG. 1A might refer to the powerline grid of a single residence or to the powerline grids of several neighboring residences (e.g., several apartments in a building).
In a configuration such as in FIG. 1A, reliable efficient communications between the nodes is usually possible only if one node transmits at a time, in order that the transmission from one node does not interfere with the transmission from another node. Therefore, there is a need for a media access mechanism that enables nodes to share the media without interfering with each other. This applies to nodes of the same network as well as to nodes of different networks.
Thus, it is desirable to have nodes that belong to different networks but share a common media be able to coexist with each other. Coexistence, i.e. media sharing, entails nodes from one network that desire to communicate with nodes from another network refraining from initiating new transmissions until the ongoing transmission is complete. Nodes from different networks, however, typically utilize different communication protocol stacks, thus preventing them from detecting and understanding each other's messages (typically within the Physical layer).
In a network incorporating prior art CSMA like media sharing, a node that desires to transmit, captures the media by the mere transmission of the data message or by transmitting before or along with the data message a predefined signal or message. As long as the media is captured, no other node is permitted to transmit.
In basic CSMA, a node that desires to transmit listens to the media to determine whether any other node is transmitting and transmits only when the media is free. A variety of prior art carrier sense mechanism have been developed including physical carrier sense and virtual carrier sense.
The principle of physical carrier sense is the direct detection of carrier signal energy. The actual transmission is continuously detected as long as it exists, hence the derivation of the term ‘carrier sense.’ This mechanism is incorporated in the IEEE 802.3 Ethernet LAN standard. It is suitable for applications where every node is capable of determining whether any other node is transmitting.
This carrier sense mechanism, however, has the disadvantage of poor reliability in noisy channels where real signal cannot always be discerned from noise. In addition, it does not permit the coexistence of nodes having different technologies. Further, the mechanism is susceptible to the hidden node problem which is described below.
Virtual carrier sense is another prior art carrier sense mechanism that is incorporated, in addition to physical carrier sense, in the IEEE 802.11 wireless LAN standard. In this scheme, the determination of the media status is logical rather than physical whereby the status of the media is determined using special control messages that specify the duration of the upcoming transmission. The control message is typically transmitted robustly at high power and using a low data rate to maximize detection by nodes.
Although this technique improves the reliability over the physical carrier sense mechanism, it still has the disadvantages of (1) not permitting the coexistence of nodes incorporating different technologies and (2) suffering from the hidden node problem.
The basic carrier sense mechanism, whether physical or logical, has several disadvantages when implemented over some types of media, especially shared powerline and wireless media. One major limitation is known as the hidden node problem and is illustrated in FIG. 1B.
Referring to FIGS. 1B, 1C and 1D, the propagation conditions over the shared media are assumed to be such that adjacent nodes, e.g., nodes A and B, B and C, C and D, D and E, are able to hear one another. Non adjacent nodes, e.g., nodes A and C, are unable to hear each other. Further, assume that the media attenuation characteristics are such that simultaneous transmissions from two neighboring nodes interfere with each other. For example, simultaneous transmissions from nodes A and C interfere at node B such that node B is not aware of the transmissions. Note that the terms ‘adjacent’ and ‘neighboring’, as used in the descriptions of FIGS. 1B, 1C and 1D, refer solely to the alphabetic order of the letters indicating the nodes (i.e. A, B, C, D and E) and to the location of the nodes depicted therein. It does not necessarily imply any relationship with the physical location of the actual nodes.
The hidden node problem is described as follows. Assume node A is transmitting to node B. Node C is unaware of the transmissions due to the propagation conditions of the example, i.e. it is ‘hidden’ from node A. Meanwhile, node C begins transmitting to node D resulting in a disturbance in reception at node B.
The virtual carrier sense with RTS-CTS attempts to solve the hidden node problem. The mechanism involves source and destination nodes exchanging a pair of special control messages (i.e. RTS and CTS), which are adapted to indicate the length of the upcoming transmissions. Any other node that detects these control messages knows that the media is going to be occupied for the time duration specified in the control messages.
Referring to FIG. 1C, in accordance with the prior art virtual carrier sense with RTS-CTS mechanism, nodes A and B exchange RTS-CTS messages. Node C detects the CTS signal generated by node B and refrains from transmitting, thus solving the hidden node problem.
Although this mechanism does solve the hidden node problem, it does not permit nodes with different technologies to coexist. Another limitation to the RTS-CTS mechanism it that there are hidden node situations that the mechanism does not solve, e.g., the example described below, which is referred to as the ‘mask node’ problem.
The mask node problem is described with reference to FIG. 1D. Assume node A is transmitting to node B and node D is transmitting to node E. Because node D is transmitting, node C does not detect the CTS message transmitted from node B. Therefore there is the danger that if node D stops transmitting while node A is still transmitting, node C might start transmitting and disturb the reception at node B. Note that the term ‘data transmission’ is intended to represent any type of transmission including data, control, etc. and is not intended to limit the type of transmission.
Note that in this mechanism full time carrier sense is replaced with signaling at the beginning of the message body only. The problem, as illustrated here, is that if a node misses the signaling for some reason, it may be unable to detect the virtual carrier signaling at a later time. In addition, the mechanism requires that all nodes implement the same communication technology in order that every node will be able to detect, decode and understand the control messages generated by other nodes.